Initial commit: CarrotAssistant backend (Go + Gin + SQLite, OpenAI-compatible agent runtime)

internal/security/crypto.go

@@ -0,0 +1,97 @@
+// Package security provides the security primitives used across the backend:
+// symmetric encryption for secrets at rest, hashing for access tokens and
+// passwords, and the prompt-injection / least-privilege controls applied by
+// the agent runtime.
+package security
+
+import (
+	"crypto/aes"
+	"crypto/cipher"
+	"crypto/rand"
+	"crypto/sha256"
+	"encoding/base64"
+	"encoding/hex"
+	"errors"
+	"fmt"
+	"io"
+
+	"golang.org/x/crypto/bcrypt"
+)
+
+// Crypto wraps an AES-GCM cipher derived from a master key. It is used to
+// encrypt model API keys at rest.
+type Crypto struct {
+	aead cipher.AEAD
+}
+
+// NewCrypto initialises an AES-GCM cipher from a 32-byte master key.
+func NewCrypto(masterKey string) (*Crypto, error) {
+	if len(masterKey) != 32 {
+		return nil, fmt.Errorf("master key must be 32 bytes, got %d", len(masterKey))
+	}
+	block, err := aes.NewCipher([]byte(masterKey))
+	if err != nil {
+		return nil, fmt.Errorf("create aes cipher: %w", err)
+	}
+	aead, err := cipher.NewGCM(block)
+	if err != nil {
+		return nil, fmt.Errorf("create gcm: %w", err)
+	}
+	return &Crypto{aead: aead}, nil
+}
+
+// Encrypt returns a base64-encoded ciphertext of the plaintext. A random
+// nonce is prepended to the ciphertext.
+func (c *Crypto) Encrypt(plaintext string) (string, error) {
+	if plaintext == "" {
+		return "", nil
+	}
+	nonce := make([]byte, c.aead.NonceSize())
+	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
+		return "", fmt.Errorf("read nonce: %w", err)
+	}
+	ct := c.aead.Seal(nonce, nonce, []byte(plaintext), nil)
+	return base64.StdEncoding.EncodeToString(ct), nil
+}
+
+// Decrypt reverses Encrypt. An empty input yields an empty output.
+func (c *Crypto) Decrypt(b64 string) (string, error) {
+	if b64 == "" {
+		return "", nil
+	}
+	raw, err := base64.StdEncoding.DecodeString(b64)
+	if err != nil {
+		return "", fmt.Errorf("base64 decode: %w", err)
+	}
+	ns := c.aead.NonceSize()
+	if len(raw) < ns {
+		return "", errors.New("ciphertext too short")
+	}
+	pt, err := c.aead.Open(nil, raw[:ns], raw[ns:], nil)
+	if err != nil {
+		return "", fmt.Errorf("gcm open: %w", err)
+	}
+	return string(pt), nil
+}
+
+// HashToken returns a hex SHA-256 digest of an app access token. Tokens are
+// random and high-entropy, so a plain hash (no salt) is sufficient and keeps
+// lookups O(1) on the unique index.
+func HashToken(token string) string {
+	sum := sha256.Sum256([]byte(token))
+	return hex.EncodeToString(sum[:])
+}
+
+// HashPassword returns a bcrypt hash of a password.
+func HashPassword(password string) (string, error) {
+	b, err := bcrypt.GenerateFromPassword([]byte(password), bcrypt.DefaultCost)
+	if err != nil {
+		return "", fmt.Errorf("bcrypt: %w", err)
+	}
+	return string(b), nil
+}
+
+// VerifyPassword checks a bcrypt hash against a plaintext password.
+func VerifyPassword(hash, password string) bool {
+	return bcrypt.CompareHashAndPassword([]byte(hash), []byte(password)) == nil
+}